The function of the nervous system relies on synaptic transmission. Synaptic transmission is mediated by calcium-triggered vesicle fusion, followed by vesicle endocytosis that recycles vesicles. Although significant progress has been made in understanding these processes, much remains unknown. My goal is to advance our understanding of these synaptic signaling processes. The progress of the last year is described below. 1. Mechanical force is needed to mediate endocytosis, whereas endocytosis may clean the active zone to facilitate vesicle mobilization to the active zone, by which synaptic transmission is facilitated. Whether actin, the most abundant force-generating molecule, is essential for vesicle mobilization is unclear. Here we addressed this issue using knockout approach combined with measurements of membrane capacitance, and imaging of vesicular protein exocytosis. We found that two actin isoforms, - and -actin, facilitate replenishment of the readily releasable vesicle pool, via endocytic clearance of active zones. 2. Vesicle fusion forms an -shaped structure (-profile), which may merge with the plasma membrane, followed by endocytosis to retrieve the fused membrane how -profile merging is mediated is poorly understood. Here, using confocal and super-resolution STED imaging, force measurements, pharmacology and gene knockout, we show that dynamic assembly of filamentous actin, involving ATP hydrolysis, N-WASP and formin, mediates -profile merging by providing sufficient plasma membrane tension to shrink the -profile in neuroendocrine chromaffin cells and at synapses. These results uncover molecular and biophysical mechanisms of -profile merging.